Radio system for transmission and reception on the same frequency



Dec. 28, 1948. M, L DOELZ RADIO SYSTEM FOR TRANSMISSION AND RECEPTION ON THE SAME FREQUENCY v 2 Sheets-Sheet 1 Filed Feb. 28, 1944 v Dec. 28, 1948. M. l.. DoELz. 2,457,134

RADIO SYSTEM FOR TRANSMISSION AND RECEPTION ON THE SAME FREQUENCY Filed Feb. 2a, 1944 2 sheets-sheet 2 n omg Patented Dec. 28, 1948 RADIO SYSTEM FOR TRANSMISSION AND RECEPTION ON THE SAME FREQUENCY Melvin L. Doelz, Cedar Rapids, Iowa, assignor to Collins Radio Company, a corporation of Iowa Application February 28, 1944, Serial No. 524,206

(Cl. Z50-13) 9 Claims.

This invention relates to a transmitter-receiver combination, and more particularly to a radio system ensuring transmission and reception on the same frequency.

One feature of this invention is that it provides an improved radio transmitting and receiving combination system; another feature of this in- Vention is that a single tuning means effects coordinated selection of a desired frequency in both the transmitting and receiving portions of the system; still another feature of this invention is that transmission and reception take place on the same frequency despite any variation of the tunable oscillator from its calibrated frequencies; yet another feature of this invention is that the system is readily tunable to any desired frequency inany of a plurality of frequency ranges; a further feature of this invention is that frequency drift is reduced; still a further feature is that undesired harmonics in a mixer output are minimixed; and yet a further feature of this invention is that it provides similar frequency spread on each of a plurality of bands and direct frequency reading on any such band. Other features and advantages of this invention Will be apparent from the following specification and the drawings, in which:

Figure 1 is a block diagram of a system em bodying my invention; Figure 2, comprising the portions 2a and 2b, is a circuit diagram of an operable embodiment of a transmitter-receiver combination system corresponding to the diagram of Figure 1; and Figure 3 is a detail view of a frequency' indicating arrangement which may be used with this system.

Two-Way radio combination is becoming of increasing importance, and under many circumstances it is desirable to have the transmitting and receiving equipment tunable over a desired frequency range, or a plurality of frequency ranges. Under such conditions the transmitted frequency cannot be crystal controlled, but must have a frequency controlled by a tunable oscillator tank circuit. Despite the greatest care in design and construction, an oscillator tank circuit undergoes frequency changes as the result of its exposure to variations in temperature, humidity, etc., and this makes it difficult to determine accurately, by any system of dial calibration, the frequency being transmitted or received, particularly in the higher frequency bands, as those used in aircraft work.

I have devised and am here disclosing a singlecontrol (in so far as tuning is concerned) radio transmitting and receiving combination system wherein transmission and reception on the same frequency are ensured. This enables an operator to tune in on another station which is transmitting, and then to go on the air himself as soon as the other station ceases transmission with full assurance that his signal is ony the same frequency as that on which the other station was transmitting, regardless of the setting of his tuning dial or the accuracy of its calibration. The system which I am here disclosing and claiming makes use of a variable generator or tunable oscillator and of a fixed frequency generator comprising a source of a plurality of fixed frequencies, both generators being operative whenever the system is in operationl and their outputs being mixed. In one case the mixed output is amplified and transmitted; inthe other case, in order to receive on the same frequency, the xed frequency generator has its frequency displaced and the resultant displaced output is used as the local or heterodyning Wave of a super-heterodyne receiver having its intermediate frequency amplier tuned to a frequency representing the difference between the initial and displaced frequencies of the mixer output..

The system which I am here disclosing and claiming` is only one system capable of ensuring transmission and reception on the same frequency provided the tunable oscillator is left at the same setting. Another system for accomplishing this purpose, and broader claims thereto, are to be found in a copending application of one Frank M. Davis, Serial No. 548,978, led August 11, 1944. Still other systems for transmitting and receiving on the same frequency with a single-control arrangement are the subject matter of a cepending application of one Arthur A. Collins, Serial No. 524,204, filed February 28, 1944, now Patent No, 2,447,490, which issued August 24, 1948, and of -another application of the said Frank M. Davis, Serial No. 524,205, led February 28, 1944, which has matured into Patent No. 2,402,606, dated June 25, 1946, and of an application of Frank M. Davis, led July 24, 1946, as Serial No. 685,934.

My system shown and claimed here comprises, as one of its principal elements, means for providing a Wave output which consists of a xed frequency generator here shown as comprising a one megacycle oscillator and a multiplier for providing any desired multiplication of such frequency; a variable generator or tunable oscillator tunable overa band having a frequency width equal to the spacing between the various fixed frequencies; and a mixer for mixing the outputs of these two generators. Transmission is effected tuned to another signal, switching to transmission ensures sending on the exact l'frquencyof the signal previously heard; and conversely, when one has been transmitting, switchingvwto receiving without touching the tuninglcritrolensuresre'- ception on the exact frequency being previously transmitted. v

While I am terming the system l a single-control system, it will be understood that by this it is meant that there would be only "a singletuning controlfor both thev transmitting 'and'receiving portions of the system, which would 'normally be 'housed' inialsingle cabinet. In'addi- "tion, :of course, there would be a band'switch, an- 'tenna" coupling wandfloarling controls (at least in high frequencyfwork) "an on-'oi switch or "sev- "eraljf"suchswitclres, anda volume control for "the receiving system. The switch-'over from disclosed here v'transmission toireception, andvi'ce versa, would 'normally be effectedby-a keying relay, or by a "push-buttono'perated Arelay associated withl the lQ'l'iOp'lflOne`"of tli'etiansnlitter. With av System "of ithisfkind, nomatter how inexperienced the operator 'may be,"`on'cehe1nds and hears'a given c'station (whether it' beA another airplane, 'a ground f' fstationfonthe like) 'he is'sure-tobe able to talk toit, s'uretob'efon that" frequency 'merely by `ef- 'fectin'gthe switchsove'r'from reception totransfinission. Anothenadvantagaclaimed in one of Y "the"a .foresai`d 'copendingDavis applicationsA (Serial `No. Y"54-8',978,' iiled `August 111,1944), islthat the jsameiantennaftuning andlo'adi'ng circuit may be 'us'edfor coupling'both-:the'transmitter and re- 'iceivertothe antenna,so"thatf the antenna can be tunedduring reception, without'breaking radio Asilence,tary/ith*assurancethat transmission will be fat Ior A'very near 'maximumpow'er without the 'necessity 'for adjustmentpfthe 'antenna-tuning and loading*controlsafter transmission has been started. v I p y In the particular `erribodiment of Amy* system 'illustrated here andfre'ferring" first ymore particularlyito th'eb'lock diagram'iligurel) a'tunable "oscillator NA 'delivers "its output vto, a `mixer `B, thisIr'ixeralso having suppliedthereto r'any of 'various predetermined lfixed frequencies here shownasg'enerate'd by 'a fixed' oscillator C, and a Lmultiplie'r D. rll'i'ewaveoutput of the mixerB iis "passed through-"a 'buffer amplie'r E and a tunabley filter circuitEthe' output of this tunable 'ltercircuit being connected to the movable element'of"atwoeposition`sWitch`FC This Vis one of a gang 'of switcheseffecting switchover from transmission 'to reception, the 'position of the "nfofvabler switch element. shown inv solid linesv 'connectingfthe'lter vcircuit totheA receiving system todeliver the'wave voutput'thereto, the connection inthe other or transmittinglposition (shown in dotted lines) 'connecting the filtercircuit` to the power amplifier. fDurin'g"transmissionthe wave `output of themixerfB "serves asa carrier'fre- :quericyfandis'ampiine'djinithe power amplifier desired Vmodulation bing'elfected'by va modurlat'o'r Any 'cor'iventiiiri'alfform of antennasstem, here show-"n"dia'g'rammauca11y, has an 'ani tenna tuning and loading arrangement J associated with it, this arrangement being connected to the movable element of another two-position switch J this movable element being shown in receiving position in solid lines and in transmitting position in dotted lines. In this latter position the modulated and amplified carrier wave is deliveredto the antenna and transmitted in conventional fashion.

When the system is switched from transmission to reception without touching the tuning control-(as by operation of the ganged throwoverswitches),the fixed frequency delivered by C andD is changed and receiving connections are E made throughoutthe system. For example, the

fixed oscillator used on transmission might have been-'generating one megacycle and the multiplier quadrupling this; the tunable oscillator may have been set at 1500 kilocycles; and the mixer might be arrangedfor subtracting, this being preferable,

"resulting Yin 'a transmitting frequency of 2500 "kilocycles y reception the'xedfrequency generator might `be When the change over was made to changed so as to deliver 5 megacycles to the mixer 'B, the change in position'of the switches F and J causing the displacedmixer wave output of 3500 kilocycles and any signals received on theantenna to be delivered to 'the' receiving portion of the system.

The receivingportion of the'system isshown schematically as vcomprising a radio frequency amplifier K; vra'mixer L having tuned circuits associated' therewith; ank intermediate frequency amplifier Mythe tuned circuits of this 'amplier being peaked'at 'a frequency of one me'gacycle underthe conditions assumed; a second detector N; and a conventional audio amplifier O'. The 3500 '-kilocycle signal delivered'y to the mixer, in

"conjunction with the vintermediate frequency amplifier circuit tuned tok one megacycle, enables the receiving portion ofthe system to .passonly delivered to the detector N and audio amplifier 0. In'or'der to achieve this desired result'infthe 'systemhere disclosed, the circuits ofthe inter- Emediate frequency amplier M must be tuned to jthe frequency representing the difference between adjacent steps or frequencies delivered by the 4fixed frequency generator here shown asfcomprising the fixed oscillator C' andthe multiplier D, although it will be understoodfthat. a plurality of' independent fixed frequency oscillators could be'us'ed rather than a single ,oscillator and a multiplier.

vForl the purpose of this r.desciriptiongit willbe assumed that thefixedvoscillaytor C deliversa xed frequency of `Aone megacycle; and thatthe 'multiplier is arranged'to.multiplyl this 5, 6, 7 orf 3 times; and that the tunable-oscillator Aisr so de- `'signed as to have its frequency .variable overa range .from one .megacycle to two megacycles. lOperation of thechangeoverswitch arrangement always displace'stheoutput of the fixed'frequency generatorlby one. in'egacycle, preferablyv raisingk yit one megacycle in each case regardless of itsfinitial fsettin'g, theinitial setting determining the band `garfd`it"will be vunderstood that the word disfaisfrnsi an increase or a decrease of frequency. For conjvenience of description,` the frequency used for transmission is being termed the se1ected, initial cr desired frequency, and the different `frequency for local heterodyning purposes is being termed the displaced frequency.

Referring now more particularly to the circuit diagram of Figure 2 (comprising portions 2a and 2b), theportions corresponding to the various rectangles of the block diagram of Figure 1 are similarly lettered. The oscillator A is shown as comprising a tube I0, which may be tube type No. 6J5, having a tank circuit comprising a permeability tuned variable inductance ll and a fixed condenser l2. While the tuned tank circuits throughout the system are shown as having fixed lcondensers (fixed except for such slight variations Aas may be desirable for trimming purposes) and variable inductances, it will be understood that the principle is equally applicable to circuits where the inductances are fixed and tuned by variable condensers. Moreo-ver, as will be more apparent later on in the description, the various tank circuits switched in changing from one frequency' band to another are shown asl completely separate and independent tank circuits, whereas in practice it may be necessary only to switch the fixed elements of such circuits.

The output of the oscillator A is here shown as loosely coupled to the signal grid of the mixer B, here shown as comprising the tube I3, which may be tube type No, 6SA'1, and its associated tuned circuits. The system here illustrated is shown as intended to provide transmission and reception on three bands, and the tuned circuits associated with the mixer tube during transmission are the three tank circuits comprising the tunable inductances Ma, Mb and Mc, together with xed condensers 15a, I5b and I5c; and those used `during reception comprise the variable inductances IBa, lBb and lc, together with the xed condensers I1a, 11b and |10. Selection of the tank circuit appropriate to the particular band is effected by the band switch elements yassociated therewith, all such band switch portions throughout the set being ganged in accordance with conventional practice; and change over from transmission to reception is effected by the switch I8, part f the change-over relay.

The injection grid of the mixer tube I3 is supplied with any of a plurality of desired fixed frequencies from the fixed frequency generator comprising the portions C and D. The fixed oscillator Cis here shown'as comprising` a tube i9, which may also be a tube type No. 6J 5, having associated therewith a tank circuit, comprising the inductance 2U and the fixed condenser 2 l, and the crystal 22, the tank circuit and crystal holding the output of the oscillator tube I9 at a fixed predetermined frequency, as one megacycle. The output of this fixed oscillator is delivered to a multiplier here shown as comprising the tube 23, which may be tubetype No. 6SJ'7, and associated tuned circuits,

certain of such circuits being used for transmission and other of said circuits for reception. Since the fixed frequency generator need only deliver four xed frequencies for proper operation on three bands, only four tuned circuits are associated with the multiplier or distorter tube 23. Three-position band switch sections, forming part of the ganged band switch arrangement of a sys'- tem, are coordinated with two-position switches forming part of the change-over relay arrangement to control selection of the circuits for transvarrangements are vvhere identified ingeneral-as 24, 25, 26 and 21. Since they are all similar -in arrangement, except for the specific values of the reactances used, only one will be described in more detail. The tuned circuit combination identied as 24, for example, comprises xed inductances 24a and 2417, these being coupled through the xed condensers 24e and 24d, these condensers each being in shunt with the respective inductances 24a and 24h through the xed condenser 24e. When the band switch is set in the first band position, as shown in the drawings, the tuned arrangement 24 is used on transmission and the tuned arrangement 25 during reception (the transmission reception change over switches being shown in this latter position) on band 2, the tuned arrangement 25 is used on transmission and 26 during reception; and on band 3 the arrangement 26 is used for transmission and 21 for reception. It Will be understood that this 'particular single-frequency fixed oscillator C' and multiplier D are used for convenience, and for absoluteA accuracy of spacing between the frequencies on transmission and reception; but that a pluralityof individually crystal controlled fixed oscillator arrangements could also'be used. Also, while the reactance elements in the tank circuits associated with the tubes I9 and 23 are spoken of as xed, it will be understood that theyv` are not tuned or varied during operation of the system; but that certain of such elements may be variable, or may have smaller, similar variable reactances in shunt or series therewith for trimming purposes.

The wave output of the mixer B is delivered to the buffer amplifier E comprising the tube 28, which may [be tube type No. 6ST?, and its associated tuned circuits here designated as the filter circuits F. There are two sets of such tuned circuits, one for transmission and one for reception, the change over being by means of the change-over switch 29. The set of tuned circuits used on transmission are here identified as 30, 3l and 32; and the corresponding set used on corresponding bands for reception are here identified as 33, 34 and 33. Each tuned circuit is here shown as comprising a. tuned inductance element (ganged with the other tuned elements throughout the system), and a xed capacitive element. The output of the buffer amplier and filter circuit in use at any particular time is adapted to be selectively connected, through operation of vthe change-over switch F', either to the first tube of the power amplifier, or to the mixer or first detector in the super-heterodyne receiving system, this latter position of the switch being that shown in the drawings.

First to follow out circuits during transmission, however, the output of the filter circuit F is then delivered through the switch F (when it is in the position other than that shown) to the rst tube of the power amplifier, which may be tube type No. 6SJ7, this tube being here identified as 36.

.` ting position (the position other than that shown) the voutput lof the power amplifier is deliveredto the antenna, the transmitted frequency being still further controlled by the tunedtank circuits .here identified as 42, 43 and 44, these'circuits also serving as the first or primary radio frequency circuits upon reception. All of the three-posi- 4ticnswitches are, of course, sections of a single band switch; and all of the two-position switches a resistance coupling, to a power modulator tube ,A6, which may be tube type No. 807. .of thisr modulator tube is transformer coupled to The output one of the grids of vthe power output tube 40 to eiect the desired modulation. It will be under- Vstoodthat the various amplifying portions heretofore described, and those to be described in :connection with the receiving portion of the system, are shown with the minimum number of tubes providing an operative arrangement, for purposes of` simplicity of illustration. In commercial practice, most `of these various amplifying sections would comprise a larger numberof tubes. For example, the power output tube 40 would normally be preceded by more than one stage of preliminary amplification, and the` same is true of the modulator tube 46; the multiplier would normally comprise at least two tubes, so

Vthat when higher orders of multiplication are desired one tube can double and the succeeding tube triple or quadruple, for example; and the radio frequency, intermediate frequency and audio frequency amplifying portions of the superheterodyne receiver portion of the system would usually comprise several tubes in cascade, rather than single tubes shown as representative.

Referring now more particularly to Figure 2a, the radio frequency amplifying portion K of the receiving system is here shown as comprising a tube 50, which may be tube type No. 6SK7, with its output tuned by one of the three tank circuits 5 l, .52 and 53, depending upon the band to which the set is tuned.` The output of this radio frequency amplifier is here shown as delivered to the signal grid of the first detector or mixer tube-54, which may be tube type No. 6SA7 and the injec- `tion grid of this tube 54 is supplied Withthe displaced wave output from the mixer B, through the Wire 55. Signals received on the antenna and which have passed the tuned circuits associated with the antenna and with the output of the tube 50 are thus beat down in the first detector tube 54, to an intermediate frequency, and this intermediate frequency is amplified in an intermediate frequency amplifier here shown as comprising only the tube 5G. Input and output coupling associated with this tube 56 is through conventional tuned intermediate frequency amplifier vcoupling circuits here identified as 51 and 58;

and inthe circuit heretofore assumed these would be tuned to pass 1,000 kilocycles and only a few kilocycles on each side of this central or peak frequency. The output of the intermediate frequency amplifier is then delivered toa detector and first audio amplifier comprising the tube 59, which may be tube. type No. 6SQ7. The output of this tube is here shown as delivered to a power or outputaudio amplifying stage .here shown as including the tube 6B, which may be tube type No. 6V6; and the output of this tube may be transformer coupled to any desired translator, as `earphones or aloud speaker. Y

If it be assumed, as heretofore, that the xed oscillator C generates a wave of a frequency `vof one .megacycla that the tunable oscillator covers .a bandof-from one to two megacycles, and that it is desired to have the three bands .covered by the sets-4, 4-5fand 5-6 megacycles, the values of the reactances inthe various tuned :circuits may be readily calculatedbyanyone skilled in the art. The values ofr inductances and capacities should beso chosen that the-tuned circuits associated with `the antenna, `the Aradio frequency amplifier .and .those vin the power amplifier, should be L -such that they are tunable, respectively, from 3'.4, 4-5, and 5-6 megacyclesyand the Vtuned circuits associated during transmission with the lmixer tube i3 land with the `output of theV buffer amplifier 28 should be similarly tunable over similar ranges. The vtank circuits associated during reception with theoutput of the mixer i3 and with the output of the buffer ampli-lier 28 should :be designed to tune over bands'of 4-5, 5-6, and 6-7 megacycles, respectively. The circuits 24, 25, `2&5 and .21 forming part of the multiplier arrangement, would, under the vconditions'assumed, be tuned to A5, 6, '7 and 8 megacycles, respectively. Other circuit constants would be of conventional value for the particular tubes used, the .present invention residing in a new concept and combination of circuit elements rather than in critical values of any particular elements of the combination.. l

I find it preferable to use subtraction rather lthan addition in achieving the desired outputsof 40 tunable oscillator has a minimum frequency less -quencies in thedesired bands. difficulty in .this regard .can mostly be eliminated than half that of the desired frequency, second, Vthirelfand fourth harmonics of the frequency `of this variable oscillator intersect or A,meet fre- I have found that bythe use of two sharply tuned circuits (preferably link coupled, as illustrated) between the variablevoscillator l0 and the mixer I3..

Even if the Amixer input derived from the variable oscillator very closely approximates a Vpure sine wave, however, the mixing action in the mixer I3 inherently creates harmonics, so that no amount -of care in the use of, sharply rtuned circuits between the variable oscillator and the mixer can completely eliminate such trouble;

and. tuned circuits subsequent to the-mixer are of little Value in eliminating the harmonic interference, since if the .undesired harmonic wave approaches' the desired signal frequency, the harmonictwave `goes through the subsequent tuned gcircuits with little attenuation.

The output of a mixer following the square law, as is the general case, has a desired youtput rproportional tothe product `ofthe two inputvoltages; the second. harmonics are proportional to the. square of the voltage vwith which it is associated (although with a different constant factor,

Aof course) and the succeeding harmonic is approximately,proportional to the cube of the voltage from which it is derived.. If thev frequency derived rfrom the rxed frequency generator is above the desired frequency in which the operal toris interested, as is preferably the case, its voltage lmay be made quite strong with no. trouble from its harmonics, vas they are always above the ,95. band vof frequencies in which the operator is interested. On the other hand, since the desired signal output is proportional to the product of the two voltages delivered to the mixer, reduction in strength'y of the voltage of the variable oscillator only reduces the value of the desired signal as a direct function while reducing the value of the second harmonic .as va square function, of the third as a cube function, etc.

Accordingly, where the fixed frequency is above the desired signal frequency'and the tunable frequency below it, as I have found to be preferable, difficulty with harmonics of the variable oscillator frequency can be minimized by making the variable oscillator voltage delivered to the mixer very small, and that delivered from the fixed oscillator many times greater, preferably one hundred or moretimes greater than the voltage of the variable oscillator. In a particular model of this system which I have built, I have found it desirable to so arrangey the couplings and powers of' the fixed and variable oscillators that one of the grids of the mixer I3 has about two volts delivered to it from the fixed` oscillator, while thel other grid has a voltage delivered to it from the tunable oscillator which is in the order -of 1/ioo of a volt. This results in the second harmonic of the tunable oscillator voltage being about '70 decibels down inthe plate current of the mixer tube with other harmonics being proportionately lowerl Even where the second harmonic frequency is exactly the 'same as the desired frequency, therefore, as happens at two or three places in the bands previously described, such second harmonic voltage is sufciently minimized that, as a practical proposition, it does not create objectionable interference; and where the desired and harmonic frequencies differ even slightly, the `subsequent tunedcircuits associated with the buffer and power amplifier attenuate the undesired signal even further.

- One every considerable advantage of spacing the various fixed frequencies by ten-to-an-integral-power cycles (as one megacycle) is that it enables the use of an improved direct reading frequency indicating arrangement enabling direct' reading of the frequency at any setting on any of the bands. Moreover, having the variable oscillator cover the same range under all circumstances and deriving the desired output in the different bands by mixing different fixed frequencies therewith results in the same frequency spread, and exactly similar tuning characteristics in each of the various bands. Moreover, combining the tunable oscillator wave with a crystal controlled, fixed frequency wave several times its frequency proportionately reduces the percentage of frequency drift which may be present in the output of the variable oscillator. The combined results provide a radio system with very desirable tuning characteristics.

The way in which the frequency to which the system is tuned may be read from the indicating means (within the limits of accuracy of the calibration, of course) is apparent from Figure 3. One indicating member, here identified as l0, is connected to and actuated by and in accordance with the band switch; and this indicating arrangement, in each particular position, shows only a single digit corresponding to the initial digit of the band frequency. That is, in the band frequencies previously assumed, the indicator 'lll would exhibit the numeral 3 when the band switch was in its first position; and the numerals 4 and 5 would be exhibited, respectively, when the band switch was in its second and third positions.

vThe indicating arrangement associated with the variable reactive element of the tunable oscillator (the variable inductance in this particular case) is .here identified as 1I This may be a' steel tape,as illustrated, or a dial or a cylinder; and it is larranged adjacent and readably following the other indicator, in' the particular case illustrated being immediately beneath the other indicator.` Moreover, the second indicator 1| should bear such numerals,fpreferably vertically arranged as illustrated, as are necessary to complete the 'frequency reading in kilocycles. In the particular example illustrated in. Figure' 3, the set is onthe second band, covering 4-5 megacycles; and the settingof the Variable element of the tunable oscillator is such'that the desired. frequency lis near the lower end of this band, 4150 kilocycles; and this isdirectly and con-A venientlyreadable` from the indicator arrangement. If the band switch is changed to the first band without Varying the setting of. the tunablev oscillator, vthe indicating arrangementr would immediately show that the frequency to which the set is tuned is 3150.ki1ocycles; whereas if the band switch had been left in the second band and t-hel tunable oscillator control changed until itsfrequency was Aexactlyin the middle of the band, the indicator would immediately show that'the frequency to which the set istuned is 4500 kilocycles. It is'obvious that such a desirable indicating arrangement can beused at any time that the variable tuning covers the same range inv each band, suc-h' range is 100, 1000, 1,000,000 lor anynumber of `cycles which is tento-an-integral-power cycles, and wherein the band switch arrangementprovides steps which are each some multiple of this number. That is, if the tunable oscillator were to cover only a range of kilocycles, the same desirable direct reading couldbe achieved by having the band switch effect band changes which are some multiples of 100. In such a case, of course, the band switch indicator would 'still provide the first digit but the second indicator arrangement would only provide two succeeding digits.

While I have shown and described certain. embodiments of my invention, it is to be under` stood that it is capableiof many modifications. Changes, therefore, in the construction and arrangement may be made without departing from the spirit and scope of the invention as disclosed in the appended claims. i

:I claim:

l; ARadio apparatus of the character described, including: a variable frequency generator.; a fixed frequency generator; means for mixing the outputs of said generators and filtering the resultant waves to provide an initial wave output; means for amplifying and transmitting said initial wave output; means for displacing the frequency of the fixed generator to effect a predetermined displacement of the frequency of the wave output; and a super-h-eterodyne receiving system having an intermediate frequency amplifier adapted to pass a frequency corresponding to the difference between the initial and displaced frequencies of said waves, a mixer delivering its output to the intermediate frequency amplifier', and means for connecting the first mentioned mixer to the last mentioned mixer, whereby the displaced waves serve as the local heterodyning waves for said receiving system.

2. Radio apparatus of the character described,vl including: a variable frequency generator; a fixed frequency generator; meansfor mixing the outputs of said generators and filtering the resultant waves to provide an initial wave output; means for amplifyingv and transmitting said initial wave output; means .for displacingthe4 frequency of the xed generator by .a given frequency to effect a displacement of the frequency of the Wave output by said given frequency; andv a super-heterodyne receiving system `having an in termediate frequency ampliertuned .to said given frequency, a mixer delivering its output to the intermediatefrequency amplifier, and means for connecting the first mentionedfmixer to the last mentioned mixer, whereby the displaced waves serve as the local heterodyning .waves for said receiving system.

.3. Apparatus of the character claimed in claim 2, wherein the variable generator is tunable over a band having a Width equal to said given frequency.

4. Radio apparatus of the character described, including: a variable frequency generator; a iixed frequency generator; meansfor mixing the outputs of said generators and iltering the resultant waves to providean initial wave output; means for amplifying and transmitting .said initial wave output; means for displacing the frequency vof the fixedl generator by a given frequency to effect a displacement of the frequency of the wave output by said given frequency; a super-heterodyne' receiving system having an intermediate frequency amplifier tuned to ,said given frequency and a mixer delivering its output yto the intermediate frequency amplifier; and` selectively operable means for connecting the mixer to the amplifying means inv one roperative positionand for actuating the displacing means andconnecting the first mentioned mixer to the mixer of the receiving system in another operative position, whereby the displaced Waves serve asthelocal heterodyning waves for ysaid'receiving system and the received frequency is identical with the transmitted frequency. r

5. Apparatus of the character claimed in `claim 1, wherein the mixer subtracts the frequency of the variable generator from vthat of the xed generator to provide said initial wave output.

6. Apparatus of the character claimed in claim 4, wherein the mixer subtracts the frequency cf1L the variable generator from that of the fixed generatorto provide saidinitial Wave output.

'7. Apparatus of the character-claimed in claim 1, wherein the voltage delivered to the mixer from the xed generator is many times that delivered from the variable generator `andthe mixer. subtracts the frequency of the variable generatorv quency` generator tunable over a band having. a.

width equal. to saidgiven frequency;y means Vfor mixing the outputs of, said generators and filter-.- ing vthe resultant waves to provide an initialwave output; means for amplifying and. transmitting.

said initial wave output; meansr lfor selecting ane,vr

other of said plurality of fixed frequencies to effec-t adisplacement of the frequency ofthe' wave output by saidgiven frequency; andgrl super-heterodyne'receivingfsystem having an in-y termediate frequency amplifier tuned to said given frequency, a mixer delivering its outputto the. intermediate frequencyampler, and means for connecting the first mentioned mixerto the last Amentioned mixer, whereby the ldisplaced waves serve as the localheterodyning waves for said receiving system. I r l l 9. yApparatus of the character claimed Ain claim 8, wherein said given frequency is ten-to-anintegral-power cycles. y. y v MELVIN L. DOELZK. Q

REFERENCES -GITED r l The following references are of record in the nle of this patent:

UNITED STATES-PATENTS y Date Numberl y Name y y 1,901,043 Roosenstein Mar. 14, 1933 1,993,395 Beers Mar; 5, 1935 2,064,961 Tidd 1 1 Dec.v 22, 1936' 2,113,419 Young Apr. 5, 1938l 2,265,983 Peterson Dec. A2,1941- 2,281,982 Leyn Mayf5, 1942` 2,317,547 McRae Apr. `27, 1943 2,401,481 Harriett June 4, 1946 OTHER REFERENCES Publication No. sfr-220, The operation of Frequency Converters and Mixers rfor Superheterodyne Reception by E. W. Herold, February 1942.

Research Laboratories, RCA Manufacturing- Company, Inc., Harrison, New Jersey, pp. 84403'. (Copy in'Div. 51.) k

Terman, Radio Engineering, l2nd ed., i' pp. 448, 449, pub. 1937 by McGraw-Hill Book Co., Inc. (Copy in Div. 51.)

Morecroft, Principles ,of Radio Communica tion, 3rd ed., pp. '769,` 770,'pub. 1933 by Johrr Wiley and Sons, Inc. (Copy in Div. 51.) l

Certificate of Correction Patent No. 2,457,134. December 28, 1948. MELVIN L. DOELZ It is hereby oertied that error appears in the printed speoiiication of the above numbered patent requiring correction as follows:

Column 1, line 36, for the Word combination read communication;

and that the said Letters Patent should be read With this correction therein that the same may conform to the record of the case in the Patent Oiice.

Signed and sealed this 24th day of May, A. D. 1949.

THOMAS F. MURPHY,

Assistant Uommssz'oner 0f Patents. 

